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The arithmetic for the resolution required to record the separated lines is given on page 79.
Depending on the magnetic strength of the sunspot, around 0.1 to 0.4T, the minimum resolution would be R=>40000.
This means at 5250A, a resolution of 5250/40000 = 0.13A....
Fulvio Mete recently managed to get some good Zeeman results using his hi-res spectrograph at around R=60000.http://www.lightfrominfinity.org/Zeeman ... Zeeman.htm